3GPP TS 36.300 defines a procedure of which a mobile station (UE) performs a handover from a source cell to a target cell as the mobile station moves in a mobile communication system. Hereinafter, this handover procedure will be described.
The mobile station receives pilot signals from individual peripheral cells of the source cell, measures their reception quality levels, and transmits information of the measured reception quality levels to a source base station (Source eNodeB) that manages the source cell.
When the source base station receives the information of reception quality levels of the individual peripheral cells from the mobile station, the source base station selects a group of cells that are to be handed over, based on the information of reception quality levels of the individual peripheral cells, and transmits a handover ready request message to the group of peripheral base stations that manage the selected group of cells.
In the event that the source base station receives a request positive reply message to the handover ready request message from the group of peripheral base stations, the source base station decides a target cell that is to be handed over and transmits a handover implementation directive message that directs the decided target cell to the mobile station.
When the mobile station receives the handover implementation directive message from the source base station, the mobile station establishes synchronization with a target base station (Target eNodeB) that manages the target cell directed by the handover implementation directive message and transmits to the target base station a reply message to the handover implementation directive message.
When the target base station receives the reply message to the handover implementation directive message, the target base station transmits a handover completion notification message to the source base station.
Now, the handover procedure has been completed.
If the mobile station has failed to establish the above-mentioned synchronization, the mobile station re-measures the reception quality levels of the individual peripheral cells, establishes synchronization with a target base station that manages a cell that has the highest reception quality level, and transmits to the target base station a reply message to the handover implementation directive message. Now, if the target base station that received the reply message is ready for handover, handover of the mobile station will be successful.
In the above-mentioned mobile communication system, however, there is a case that when the source base station has transmitted the handover implementation directive message, the reception quality level of the target cell directed by the handover implementation directive message will have decreased to a level such that the base station cannot perform a handover. In addition, there is a case in which, if the mobile station fails to establish synchronization, the target base station that received the reply message to the handover implementation directive message will not be ready for handover. Thus, there is a problem in which the likelihood that the mobile station succeeds in handover decreases.
Moreover, in the above-mentioned mobile communication system, if the above-mentioned establishment of synchronization fails, the connection may be delayed due to the exchanges of unnecessary signals, and an increased load may be placed on a base station that is in the state of congestion.    Non-patent document 1: 3GPP TS 36.300, V8.4.0